FIG. 5 is a cross-sectional view showing a structure of a visible light semiconductor laser. In FIG. 5, reference numeral 1 designates an n type GaAs substrate. A double-hetero-junction structure comprising an n type (Al.sub.O.7 Ga.sub.O.3).sub.0.5 In.sub.O.5 P lower cladding layer 2 of 1.0 micron thickness, an undoped Ga.sub.O.5 In.sub.O.5 P active layer 3 of 0.08 micron thickness, and a p type (Al.sub.O.7 Ga.sub.O.3).sub.0.5 In.sub.O.5 P upper cladding layer 4 is disposed on the substrate 1. The upper cladding layer 4 has a stripe-shaped ridge part in a longitudinal direction, i.e., along the laser resonator, at the center portion of the laser in the width direction. The upper cladding layer 4, other than the ridge part, that is, the portion sandwiched by active layer 3 and current blocking layer 6, is 0.3 micron thick and the ridge part is 1.0 micron thick. A p type Ga.sub.O.5 In.sub.O.5 P buffer layer 5 of 0.1 micron thickness is disposed on the ridge part of upper cladding layer 4. A p type GaAs cap layer 8a of 0.3 micron thickness is disposed on the buffer layer 5. An n type In.sub.t Ga.sub.1 -tAs.sub.u P.sub.1-u current blocking layer 6 of 1.1 microns thickness is disposed on the upper cladding layer 4, burying the ridge 11, which comprises the cap layer 8a, the buffer layer 5, and the ridge part of upper cladding layer 4, but not on the top surface of cap layer 8a. The width of the bottom of ridge 11 is 4 microns. A p type GaAs contact layer 8b of 3.0 microns thickness is disposed on the current blocking layer 6 and the cap layer 8a. A p side electrode 9 is disposed on the contact layer 8b and an n side electrode 10 is disposed on the rear surface of substrate 1.
This structure operates as follows.
When a forward direction voltage is applied between the p side electrode 9 and the n side electrode 10, current flows only through the ridge 11 because of the existence of current blocking layer 6. Since the lower cladding layer 2, the active layer 3 and the upper cladding layer 4 constitute a double-hetero-junction structure, a portion of active layer 3 in the vicinity of ridge 11 emits light because of the current injected through the ridge 11, resulting in laser oscillation.
A method for manufacturing the double-hetero-junction semiconductor laser of FIG. 5 is illustrated in figures 6(a) to 6(f). In these figures, the same elements are given the same reference numerals as in FIG. 5, and reference numeral 12 designates a silicon nitride (SiN) film.
As shown in FIG. 6(a), the n type (Al.sub.O.7 Ga.sub.O.3).sub.0.5 In.sub.O.5 P lower cladding layer 2, the undoped Ga.sub.O.5 In.sub.O.5 P active layer 3, the p type (Al.sub.O.7 Ga.sub.O.3).sub.0.5 In.sub.O.5 P upper cladding layer 4, the p type Ga.sub.O.5 In.sub.O.5 P buffer layer 5 and the p type GaAs cap layer 8a are successively grown on the n type GaAs substrate 1 by MOCVD (Metal Organic Chemical Vapor Deposition). A typical growth temperature is 650.degree. to 700.degree. C. AsH.sub.3 gas is introduced into the reactive tube while heating the GaAs substrate 1 in order to prevent thermal decomposition of substrate 1. When crystal growth starts, the supply of AsH.sub.3 gas is stopped and a dopant source gas for AlGaInP series semiconductors such as PH.sub.3, trimethylaluminum, triethylgallium, trimethylindium or the like are introduced. After the layers from lower cladding layer 2 to buffer layer 5 are grown, the supply of PH.sub.3, trimethylaluminum and trimethylindium is stopped and AsH.sub.3 is again introduced to grow the p type GaAs cap layer 8a.
Then, as shown in FIG. 6(b), the SiN film 12 of a stripe configuration is formed on the p type GaAs cap layer 8a. Thereafter, as shown in FIG. 6(c), the cap layer 8a, the buffer layer 5 and the upper cladding layer 4 are etched away using the SiN film 12 as a mask to form the ridge 11. Then, as shown in FIG. 6(d), the n type In.sub.t Ga.sub.1-t As.sub.u P.sub.1-U current blocking layer 6 is selectively grown using the SiN film 12 as a mask. While heating the substrate on which the ridge 11 is formed and growing the n type In.sub.t Ga.sub.1-t As.sub.u P.sub.1-u current blocking layer 6, PH.sub.3 gas is introduced to apply phosphorus pressure to the substrate, so that the (Al.sub.O.7 Ga.sub.O.3).sub.0.5 In.sub.O.5 P upper cladding layer 4 does not decompose. After the current blocking layer 6 is grown, the SiN film 12 is removed and the p type GaAs contact layer 8b is grown as shown in FIG. 6(e). During heating the substrate before the growth of p type GaAs contact layer 8b, PH.sub.3 gas is introduced to apply phosphorus pressure, so that the n type In.sub.t Ga.sub.1-t As.sub.u P.sub.1-u does not decompose. When the p type GaAs contact layer 8b starts growing, the supply of PH.sub.3 gas is stopped and triethylgallium and AsH.sub.3 gases are introduced to form the p type GaAs contact layer 8b. A typical growth temperature of the p type GaAs contact layer 8b is 650.degree. to 700.degree. C. In addition, Zn is used as the p type dopant source gas and diethylzinc (DEZn) is used as p type dopant source gas. The charge carrier density of the p type GaAs contact layer 8b is controlled by controlling the flow rate of DEZn. Finally, as shown in FIG. 6(f), the p side electrode 9 and the n side electrode 10 are deposited to complete the laser structure of FIG. 5.
In this visible light semiconductor laser device, p type GaAs is used for the contact layer 8b and this induces the following problems.
Since the p type GaAs contact layer 8b is thick, that is 3 microns, it about an hour to grow that. In addition, since the growth temperature is high as 650.degree. to 700.degree. C., the activation rate of dopant impurities in the cladding layer varies or the dopant atoms diffuse from the cladding layer into the active layer during the growth of p type GaAs contact layer, resulting in deterioration in laser characteristics. In order to reduce the deterioration, it is necessary to lower the growth temperature of contact layer 8b. Here, the charge carrier density of contact layer 8b is required to be approximately 10.sup.18 to 10.sup.19 cm.sup.-3 by controlling the flow rate of DEZn, p type dopant source gas. However, when the p type GaAs is grown at a lower temperature, the introduction ratio of Zn as p type dopant into the p type GaAs must be higher, resulting in difficulty in controlling the charge carrier density. As a result, excess dopant impurity atoms enter into the contact layer and induces harmful influences such as crystalline defects. Accordingly, it is difficult to grow the GaAs contact layer at a low temperature.
Since the In.sub.t Ga.sub.1-t As.sub.u P.sub.1-u current blocking layer 6 occupies the large part of the wafer surface before the p type GaAs contact layer 8b is grown, PH.sub.3 gas is introduced while heating the substrate before the growth of p type GaAs contact layer 8b to apply phosphorus pressure, so that the In.sub.t Ga.sub.1-t As.sub.u P.sub.1-u does not decompose. On the other hand the contact layer comprises GaAs and the p type GaAs cap layer 8a having the same composition as the contact layer is disposed on the top surface of ridge 11. When the temperature rises in PH.sub.3 atmosphere, arsenic escapes from the p type GaAs cap layer 8a and then crystalline defects are likely to be created in the cap layer 8a. Since the p type GaAs cap layer 8a at the top of ridge 11 is only 0.3 micron thick, the crystalline defects are produced not only in the cap layer 8a but also in the (Al.sub.O.7 Ga.sub.O.3).sub.0.5 In.sub.O.5 P upper cladding layer and the Ga.sub.O.5 In.sub.O.5 P active layer. As a result, the lifetime of the laser is reduced.